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The atomicity property requires that all operations of a transaction should be completed if not, the transaction will be terminated. A transaction is considered as one. Therefore, it must execute and complete each operation in its logic before it commits changes. The atomicity property of a transaction is ensured by the transaction recovery subsystem of a DBMS. If a transaction fails to complete for some reason such as a system crash during a transaction execution, the recovery techniques undo any effects of the transaction on the database (Singh, S., 2010)
When a database recovers from a crash, it should:
There are two types of techniques, which can help a DBMS in recovering as well as maintaining the atomicity of a transaction:
Execution of a transaction must leave a database in either its prior stable state or a new stable state that reflects the new updates made by the transaction. If the transaction fails, the database must be returned to the state it was in prior to the execution of the failed transaction. If the transaction commits, the database must reflect the new changes. Therefore, all resources are always in a consistent state. The consistency of a DBMS is the responsibility of both the DBMS and the application developers. The DBMS can ensure consistency by putting into effect all the constraints that have been particularly on the database schema such as integrity and enterprise constraints. The log component of the recovery sub-system stores the information needed to undo the updates performed by a transaction in case it must be rolled back, either to an internal save point or to the beginning. (Liu, L., M. Tamer Ozsu, 2009)
Isolation property of a transaction means that the data used during the execution of a transaction cannot be used by a second transaction until the first one is completed. Therefore, this property isolates transactions from one another. If a transaction is using a particular data item, that data cannot be accessed by another transaction.
The isolation property is enforced by the concurrency control subsystem of the DBMS which is meant to coordinate simultaneous transactions while preserving data integrity. At the end of a transaction, either the transaction will complete successfully or not. If the transaction completed successfully, the state of the database will have changed, and the system log will have a record of the activities done. If the transaction will have failed, the database will not have changed and therefore the recovery-subsystem will use the system log to roll back and recover from any changes attempted by the transaction.
The durability property of a transaction shows the performance of a database. Changes made by a transaction are permanent. They cannot be lost by a faulty transaction nor system failure. When a transaction is completed, the database reaches a consistent state and that state cannot be lost, even if a system failure occurs. The recovery sub-system stores the content within the DBMS and ensures that the data cannot obscure.
Deadlock – A system is deadlocked if there is a set of transactions such that every transaction in the set is waiting for another transaction in the set.
Neither transaction can continue because each transaction in the set is on a waiting sequence, waiting for one of the other transactions in the set to release the lock on an item.
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